Republican candidate Donald Trump won the endorsement on Friday of New Jersey Governor Chris Christie, who said the billionaire businessman had the best chance of beating Democrat Hillary Clinton in the Nov. 8 presidential election.

“The best person to beat Hillary Clinton in November on that stage last night is undoubtedly Donald Trump,” Christie told a news conference a day after a Republican candidates’ debate. He spoke at a news conference before a Trump rally in Texas.

Donald Trump is “rewriting the playbook,” said Christie, 53, who until earlier this month was a Trump rival for the Republican Party nomination.

It was Trump’s biggest endorsement from a mainstream Republican to date. The front-runner in the race, Trump, 69, has never held public office and has campaigned as a political outsider.

The endorsement of the New Jersey governor gives Trump, from neighboring New York, a lift ahead of next week’s crucial Super Tuesday nominating contests.

Trump, a brash former reality TV show host, has now won three straight nominating contests in New Hampshire, South Carolina and Nevada, convincing some mainstream Republicans that he may have too much momentum to be stopped, especially if he wins big in the key southern U.S. primaries on Tuesday.

Former Secretary of State Hillary Clinton is battling U.S. Senator Bernie Sanders of Vermont for the Democratic nomination.